The present invention relates to a mechanical sensor for measuring dynamic quantities such as pressure, load, acceleration and displacement.
There has been a mounting demand in recent years for means to measure various dynamic quantities to improve the efficiency and function of facilities for air condition, electric instrumentation, housing equipment, electrification, factory automation and so on. The objects of measurement are varied from the load in a structure to the fluid pressure of gas or liquid, and a mechanical sensor which is applicable to many different measurements is sought for.
Heretofore, dynamic quantity measurement has been taken using a device such as a silicone semiconductor-type sensor and a strain gauge-type sensor employing a Cu--Ni alloy. These sensors are low in reliability and high in price. In addition, these can not every well be applied to wide use.
As replacement, there has been proposed a mechanical sensor comprising an insulating substrate, and a piezoresistance element and electrodes formed on the surface of the insulating substrate. When a stress is applied on the mechanical sensor, the substrate is deformed and the piezoresistance element which has been formed on the substrate is also deformed. A resistance value of the piezoresistance element changes as the length or the sectional area of the piezoresistance element changes. Therefore, the quantity of the applied stress is detected by reading a change in resistance value of the piezoresistance element. The sensitivity of such mechanical sensor is defined by gauge factor (hereinafter referred to as GF) representing the percentage of the change in resistance to the quantity of the strain as given below. ##EQU1##
The substrate made of ceramics such as alumina has been used for the mechanical sensor.
In recent years, a metal-core substrate employing a metal base coated with an electric insulating layer on the surface thereof (an enameled substrate) which are excellent in elasticity and workability has come to be widely used. As the metal base, a stainless steel sheet or a cold rolled carbon steel sheet is employed for example. As the electric insulating layer formed on the metal base, a crystallized glass of SiO.sub.2 --B.sub.2 O.sub.3 --CaO--MgO system is used, for example.
The piezoresistance element generally has a structure wherein a powder of an electroconductive material such as ruthenium oxide is dispersed in the glass. The piezoresistance element is formed by applying and baking on the substrate a mixture paste of the electroconductive material powder, a glass powder and an organic binder.
A protection layer is provided on the surface of the sensor thus obtained as necessary.
However, any of the mechanical sensors employing the piezoresistance element materials and metal-core substrate has failed to have a sufficient sensitivity to be used in various applications so far.
As the conceivable causes which affect the strain-sensitive characteristics of the mechanical sensor, interface reactions between the electroconductive materials and the glass in the piezoresistance element and reactions between the components in the piezoresistance element and the substrate components can be considered. These points are now subjected for extensive researches to improve the strain-sensitive characteristics (GF) or the sensitivity of the sensor. For example, a thicker piezoresistance element is studied to lower a ratio of the portion where the components are assumed to react with the substrate components in the piezoresistance element. In this method, thicker the piezoresistance element, higher the strain-sensitive characteristics of the sensor. However, formation of the thicker piezoresistance element causes foaming on and inside the piezoresistance element which results in an increased variation in sensor characteristics as shown in FIG. 6.